High efficiency linear amplifiers such as Doherty amplifiers are well known to those skilled in the art. However, it is also well known that Doherty amplifiers typically have relatively poor linearity. In addition, their linearity is typically inversely proportional to their efficiency. As a result, although Doherty amplifiers can improve the performance of high peak-to-average ratio linear amplifiers, they may only do so over a narrow dynamic range.
At least one attempt has been made to design a Doherty amplifier which can operate with high efficiency over a wider dynamic range by nesting Doherty amplifiers, and is described in "Efficiency of Doherty radio frequency (RF)-power amplifier systems", F. Raab, Green Mountain Radio Research Company, RN84-23, August, 1984. Nesting Doherty amplifiers as described in the Rabb implementation requires paralleling additional amplifier stages and their associated drive control circuitry.
Doherty type amplifier circuits would become more attractive if their efficiency were raised at low output power levels. In particular, in multicarrier power amplifier applications for cellular base station equipment in which the RF input signal applied to the multicarrier power amplifier may vary by as much as 30 dB.
Therefore, a need exists for a system and method for amplifying a signal in which the efficiency of a Doherty amplifier is increased over an extended dynamic range.